The "layer of Bondo" after fibreglassing isn't really a "layer of" Bondo. You only use it if and where necessary to detail the shape of the model and it serves no structural purpose. The fibreglass carries all the weight, and the resin holds it together. Rondo on the other hand is basically just resin, which is a lot weaker without fibres, so you need a lot more. Read up on reinforced plastics if you don't understand the principle.
The benefit for you in using Rondo is that it's easier to work with, throwing in some additional fibreglass (which is the difference between rondoing and mudglassing) pretty much removes all the advantages and gives you a combination of the disadvantages.
Yes, fibreglass is necessary for mudglassing, that's why it's called mudglassing. You can use Rondo without adding fibres, but then it's not called "mudglassing".